1. Technical Field
The present invention relates to a thermal treatment apparatus for objects to be treated, including semiconductor wafers and other substrates.
2. Description of Related Art
During the process of fabricating objects such as semiconductor wafers, various types of thermal treatment apparatus are used for subjecting the semiconductor wafers, which are objects to be treated, to treatments such as chemical vapor deposition (CVD), diffusion, oxidation, or annealing. For CVD or thermal treatments under reduced pressures that are comparatively high, a thermal treatment apparatus (also called a vertical thermal treatment apparatus) is generally used, wherein a gas introduction portion and an exhaust portion are provided in a lower portion of a vertical reaction tube; semiconductor wafers held in a multi-layer stack within a wafer boat, which is a means for holding semiconductor wafers, are accommodated and sealed into a treatment region within this reaction tube; the treatment region is heated to a predetermined treatment temperature by a heater provided outside the reaction tube; and the semiconductor wafers are subjected to the predetermined thermal treatment at a predetermined treatment pressure, using the predetermined treatment gas.
Since the manifold is separated from the heater in such a thermal treatment apparatus and there are problems concerning the thermal durability of sealing members, such as O-rings, that form a seal between the manifold and the reaction tube, it is usual to cool the vicinity of the sealing members of the manifold. This cooling brings the temperatures of the inner surfaces of the manifold down to well below the treatment temperature. Thus components of the treatment gas condense onto the inner surfaces of the manifold in regions outside the treatment region within the reaction tube when they come into contact therewith, reaction by-products (also called deposits, with diffusion processing) adhere to the inner surfaces of the manifold, and particles are generated. This has an adverse affect on the treatment and, moreover, regular cleaning becomes necessary. In addition, in a process that uses a treatment gas that is toxic, such as arsenic, the presence of these reaction by-products would inevitably affect the health of the operators adversely, making countermeasures necessary.
In order to address this problem, a method of sublimating reaction by-products is used in a conventional thermal treatment apparatus, wherein the vapor pressure of the reaction by-products is increased by heating the manifold from the outside.
However, since a special heater must be attached to the outer side of the manifold of such a thermal treatment apparatus, this causes a number of problems such as the apparatus itself becomes complicated, it is difficult to heat all the parts of the manifold to the desired temperature, it is necessary to consider safety problems such as burn injuries, and the necessary increase in electricity consumption adversely impacts the environment. In processes using treatment gases that contain components such as arsenic compounds, furthermore, the vapor pressure of the arsenic that is a reaction by-product is low so that the above described heating method is not greatly effective in suppressing the deposition.
To solve this problem, a vertical thermal treatment apparatus such as that disclosed in Japanese Patent Application Laid-Open No. 10-223538 (published Aug. 21, 1998) has been proposed. This vertical thermal treatment apparatus is provided with a single cylindrical reaction tube that is closed at an upper end and open at a lower end, and a thermal treatment region is defined within this reaction tube. The configuration is such that objects to be treated by the thermal treatment are conveyed into and out of the interior of the cylindrical reaction tube, through the open lower end thereof. A known manifold is provided under the lower portion of the reaction tube, and a gas introduction portion for the treatment gas and an exhaust portion are provided in this manifold. The configuration is such that a gas supply pipe is provided to extend from the gas introduction portion to an upper portion of the cylindrical reaction tube, so that the treatment gas is supplied downward from the vicinity of the closed upper end of the cylindrical reaction tube towards the thermal treatment region. A cylindrical cover is provided on the inner side of the manifold, with an annular exhaust space being formed between the manifold and this cover. The treatment gas, which was supplied so as to flow downward through the thermal treatment region and subject the objects to be treated to the thermal treatment, enters the exhaust space and is exhausted through the exhaust portion. An inert gas supply pipe is connected to a lower side of the cover, an inert gas that is sent from this inert gas supply pipe into the manifold is made to flow upward through an annular gap formed between the cylindrical cover and an insulating cylinder disposed on the inner side thereof, preventing the thermal treatment gas that is flowing downward through the thermal treatment region from flowing into that annular gap, and this prevents components of the thermal treatment gas from coming into contact with the inner side of the manifold, thus preventing the adhesion of reaction by-products thereon.
The above described known vertical thermal treatment apparatus is certainly effective in preventing the adhesion of reaction by-products onto the interior of the manifold, but it has a defect in that, because the inert gas must be made to flow upward from the inert gas supply pipe against the resistance of the downflow of the thermal treatment gas flowing through the thermal treatment region, it is necessary to increase the speed of flow of the inert gas and, as a result, the flowrate of inert gas must be increased. If the flowrate of the inert gas is increased, a large quantity of the inert gas will mix with the thermal treatment gas in the thermal treatment region, adversely affecting the quality of the thermal treatment.
The present invention was devised in order to solve the above described problem and has as an object thereof the provision of a thermal treatment apparatus which is, of course, capable of restraining the adhesion of reaction by-products on portions around the furnace entrance within the manifold, but which also enables a reduction in the flowrate of inert gas, thus reducing the likelihood of inert gas mixing with the thermal treatment gas within the thermal treatment region while having no adverse effect on the quality of the thermal treatment. Another object of this invention is to provide a thermal treatment apparatus that ensures there is no peeling of any reaction by-products that happen to adhere thereto.